Add support for compiling Storyboard files (Xcode)

This goes into the ib module. Trivial to implement:

• setenv IBSC_MINIMUM_COMPATIBILITY_VERSION = cpp.minimumIosVersion
• FileTagger for *.storyboard => ["storyboard"]
• Rule to compile ["storyboard"] to *.storyboardc

The rest is pretty much exactly the same as for xib/nib files. Note that *.storyboardc - "Interface Builder Compiled Storyboard Document" - is actually a directory containing an Info.plist and several compiled nib files, the number of and names of which are non-static.

Sample invocation:

ibtool --target-device iphone --target-device ipad --errors --warnings --notices --module Silverlock --minimum-deployment-target 8.0 --output-format human-readable-text --compile Main.storyboardc Main.storyboard